* Dalian iron ore, coking coal, coke slump by 6 pct limit
* Dalian doubles fees on coking coal, coke futures from Thursday
* Announces more new rules to come into effect from April 29
* Chinese exchanges have raised transaction costs after surge (Adds latest changes by Dalian exchange)
By Manolo Serapio Jr and Ruby Lian
MANILA/SHANGHAI, April 27 (Reuters) - A major Chinese commodities exchange took further steps to calm volatile markets on Wednesday, hiking transaction fees and widening trade limits in a move that could make exiting futures contracts more orderly.
Iron ore and steel futures fell again in reaction to higher trading costs, brought in to deter speculative investors believed to be behind last week's spike in prices and volumes that had stoked fears of a destabilising crash.
The Dalian Commodity Exchange said it would double the transaction fees on steelmaking raw materials coking coal and coke futures from Thursday, the fourth increase in a week.
It later announced that from April 29 it would further widen the trading limit for coking coal and coke futures to a 7 percent move in either direction from 6 percent, a step that could potentially reduce trading disruptions.
The exchange also said the minimum margin for both contracts would be increased to 9 percent from 8 percent previously.
Dalian had also raised transaction fees on iron ore futures twice this week as the exchange, along with other commodity platforms in Shanghai and Zhengzhou, imposed curbs to restore calm to markets after a week-long surge unsettled global investors.
The curbs implemented by the exchanges appeared to be having the desired effect, said Wang Di, analyst at CRU consultancy in Beijing.
"What they're trying to do is minimise too much speculation, especially from the retail investors."
Analysts said speculators were betting that a rise in infrastructure spending in China would lift raw material prices, which have been depressed for years by a persistent supply glut.
The rapid price gains have defied the supply-demand balance of the underlying commodities, and analysts warned there was a risk of a bigger correction ahead, similar to last summer's crash in Chinese shares that also followed a rapid run-up.
"If prices in China go too far off from fundamentals, markets in China could lose credibility, similar to what had happened in the stock market," Citigroup analysts said in a report.
The most traded September iron ore contract on the Dalian Commodity Exchange dropped by its 6 percent downside limit for a second day in a row on Wednesday, before closing 5.4 percent lower at 434.50 yuan ($66.95) a tonne.
Coking coal and coke on Dalian also slid by the 6 percent maximum allowed by the exchange, but coke slightly trimmed losses to end down 4.9 percent.
On the Shanghai Futures Exchange, rebar - or reinforcing bar used in construction - fell 3.7 percent to 2,500 yuan a tonne . The less-traded hot rolled tumbled by its 6 percent downside limit before closing 4.6 percent lower.
After raising the transaction fee on iron ore futures contracts twice, the Dalian exchange said it "will step up supervision and resolutely curb signs of overheated speculation in some products to prevent risks and maintain steady operations of markets".
Wednesday's pullback was concentrated on steel and steelmaking futures, but other commodities also slipped. Cotton on the Zhengzhou Commodity Exchange and egg futures on Dalian each fell nearly 2 percent. ($1 = 6.4901 Chinese yuan)
Additional reporting by Beijing Monitoring Desk; Editing by Will Waterman and Mike Collett-White